Given a gamma radiation field coming from specific or extensive sources (fixed or mobile) located in an open or closed space, the actual imaging of the spatial distribution of those sources of radiation is performed using the technology known as “gamma cameras”, which are available off-the-shelf.
As detailed by Kharafi in the year 2013, current gamma cameras use two types of technological approaches to imaging.
One kind of technological approach uses a segmented collimator consisting of the parallel overlay of a plurality of collimators and a certain focal point and high efficiency collimation, i.e., only radiation from a small range of angles of incidence enters the detector. The axis of each collimator converges at a certain imaginary point in front or behind the camera, creating a sort of focus of the gamma radiation. This is a position-sensitive gamma detector; this type of collimator is called “telescope”.
The other kind of approach to create the gamma image uses the position-sensitive gamma detector, inside a shielding, with a single collimator with a small entrance hole at a relatively large distance from the gamma sensor or sensors, called “pin-hole” collimator. Through the pin-hole of the collimator, each position of the sensor receives the radiation coming from a small range of angles of incidence.
FIG. 1 presents a view of a simplified diagram of both types of gamma camera devices.
Recent equipment proposed with this type of collimators can be seen, respectively, in the proposals by Enghardt et al. in 2011 and by Le Goaller in 2010.
As we can see, these cameras create the image concept making each gamma sensor have a solid detection angle towards the focal point of the gamma camera, which must be smaller as the desired spatial resolution increases. In these cameras, said resolution is high, but the detection efficiency is low because by using collimators the image is created at the expense of removing the detection of gamma radiation not aligned with the focal point, which it does that for a variety of applications the typical measurement time can be up to 10 minutes, and even more, to obtain a single complete gamma image.
Since collimators can be expensive and bulky for a variety of applications, improvements have been proposed, such as the so-called coded aperture collimators, which reduce the size of the collimator and, in turn, increase spatial resolution of the image of the gamma field. These collimators have evolved since the first proposals, such as that by Barrett in 1973, to present day when there are several gamma cameras available off-the-shelf with coded collimators, as detailed by Ivanov et al. in 2010. In open fields and subject to tests under real conditions, measurement times of more than 10 hours have been reported for a single image, as reported by the Department of Energy of the United State in 1998, applying a coded aperture camera in a certain nuclear site being dismantled.
Other methods to obtain an image, but without using collimators, as proposed by Schönfelder et al. in 1973 to investigate gamma rays for astronomical investigation purposes, which has also been developed through time for medical and scientific uses, are those using a consecutive series of detectors. The first detector performs Compton interaction, and the second works as a detector of the absorption of the outgoing gamma after Compton interaction, reconstructing the path of the incident photon in the first detector based on the separation of the signals by time of flight from both detectors, and the lobular angular response of the outgoing gamma in relation to the angle of the incident radiation. Currently, with this concept, more compact designs are yet to be achieved to use as general purpose gamma cameras, as analyzed by Ljubenov et al. in 2002, and proposed by Wonho Lee et al. in 2010, as well as by Kataoka et al. in 2013. This type of sensor has the disadvantage that the detection efficiency will be comparatively lower because it relies on simultaneous measurement of two gammas.
Although there are gamma sensor with high spatial resolution, based on the foregoing it is finally clear that general purpose gamma cameras will be slow to acquire an image in the open field, due to the low efficiency involved, and the fact that the images are blurry because the focus mechanisms used have low precision. This has been a restriction for their current use, since the equipment available are expensive, bulky and heavy, and with inconvenient features and pricing for a large number of applications.
A fast alternative to characterize a gamma field is to use several opposite detectors in quadrant arrays, separated by shielding material, such as lead or tungsten, as you can see in the proposal by Larsson et al. of 2008. Although this type of gamma detector is fast and efficient, it does not generate an image of the gamma radiation from a given field of view; instead of being a kind of gamma camera, it is a detector, by quadrants, of the measurement of the anisotropy of the gamma field.
Based on the analysis of the characteristics of current gamma cameras, these devices can be explained as concepts looking to resemble the image construction method of the human eye or current optic cameras, which are essentially based on the concept of a focus, a lens, and a position-sensitive high resolution sensor.
This technological approach to imaging (outlined in FIG. 2), has the advantage that it can be used in several of these equipment: high resolution position-sensitive detectors are already available for X-ray and Gamma ray image transmission equipment for medical and industrial applications.
But for X and Gamma ray detection with transmission technique, the source of radiation are either specific or parallel beams, which generate images with very little blur without requiring telescope or “pin-hole” collimators. To acquire gamma images from extensive sources in open spaces, the high resolution of these sensors, given the number of pixels or two-dimensional elements available to compose an image is unnecessarily high, because, due to the properties of the gamma field, changes in spatial and angular components of the actual gamma field are very small, and the image is finally very blurry, regardless of whether it is acquired with high resolution of the gamma sensor. It can be observed that this effect still appears even using the different types of collimators already mentioned, with times of up to dozens of minutes to produce one single image.
That is why there are some detectors available which are extremely slow but relatively small and low-cost, not always included in the classical literature of gamma cameras. These use a “pin-hole” collimator with an extremely small aperture, which has a single detector that is not position-sensitive, located inside the shielding and collimator, and creates the image using mechanical azimuth recording and elevation of the axis of view of the measuring head consisting of the detector and the collimator. Although they are lightweight, the measuring time for one position only can be up to 2 hours.
On the side with the longest acquisition time for gamma imaging by moving a single detector are aerial equipment with radiometric characterization for geophysical purposes, which use a single non-collimated detector with approximately 16 liters detection volume, consisting of 4 detectors in parallel, as outlined in FIG. 3. Combining commercial detectors with a 4 inches×4 inches×16 inches detection volume, they form a flat area of detection of approximately 40 cm by 40 cm sides, and an approximate 100 Kg weight for the full detector. An example of such application can be found in the report by Paasche et al. of 2003, where an image or map of gamma radiation of an area of approximately 8000 Km2 was acquired, flying 150 meters above ground in straight lines separated by 400 m over 31 flights during a month and half using two of the detectors described for geophysical measurements. This imaging method with a single sensor is also distinct because, as it is not collimated, its maximum efficiency is on the vertical over the center of the detector face, and the efficiency decreases when deviating from the regular incidence due to the decrease in geometric efficiency, as reported by Itadzu et al. in 2000. By operating as a single detector with a very large field of view and increasing height to measure the area with fewer flight hours, since the distance can be increased between flight lines, it reduces the spatial resolution of the image acquired, which is explained in detail in technical document 1363 of the International Agency of Atomic Energy of 2003.
Recently, for the purpose of searching for radioactive sources in the open field, as presented by Halevy et al. in 2014, or mapping radiation on lands adjacent to a nuclear station after an accident, as presented by Torii et al. in 2015, geophysical sensors started being used in aircrafts, helicopters and unmanned aerial vehicles. But then problem remains that the shortest measurement time requires a greater height at the expense of losing spatial resolution of the gamma image or map. Since these applications are most useful when the measurement is faster, this type of detectors present many limitations when used. The gamma cameras mentioned above cannot be applied because the measurement time is too long for a moving vehicle, whether by air or land.
In gamma cameras, collimators are necessary to create an image because, unlike visible light radiation lengths, there is no equivalent for the refraction index when transferring the gamma radiation from one medium to another, but rather the gamma radiation carries out Compton interaction, photoelectric and pair generation based on the function of the incident radiation energy, with a constant of output angles with respect to the incident radiation angle. Due to this difference in interaction depending on whether it is a visible or gamma photon, the concept of inbound and outbound beam with its angle of incidence and output is lost, which means that there is no concept directly similar to the concept of lens and focus in terms of gamma radiation.
In geophysical gamma detectors, the need to use non-collimated large detectors is due to the fact that when combining the low intensity of natural radiation with the flight speed of aerial vehicles, high detection efficiency is necessary to obtain statistics to generate the maps required in a matter of seconds.
Given the very numerous and varied needs of fast handling control of radioactive sources, safety of nuclear facilities, protection of sites, materials and people access and exit control, simple area monitors which only measure the rate of incident gamma on the detector in real time are still used, usually with Geiger counters. These detectors are widely used due to their low price, and because they have a compact and lightweight structure, and are also simple and robust.
For nuclear accidents, radioactive sources searches, or to perform geophysical gamma maps of gamma radiation, large sodium iodide scintillator detectors are still currently used as sensors, with no shielding, to transport a large volume of detectors with minimum weight.
It is clear then that it would be much more advantageous to have area monitors to perform not only the simple count of photons of gamma radiation incident on the detector, but also to create an image of the spatial distribution of the gamma radiation incident on the detector, so as to significantly contribute to detect the source of radiation. It would also be useful to perform a spectrometry, but currently existing gamma cameras are far behind as regards speed, price, volume and weight in order to compete with the everyday use of a simple Geiger counter used as an area monitor.
It is also clear that it would be much more advantageous to have meters for open images of gamma fields, which, without significantly compromising weight and volume of the equipment, can provide images in short measuring times. But, usually, there is only the alternative of using a single detector consisting of an array of scintillators.
Then, there is a real need for a device capable of performing gamma images with high efficiency within short periods of time, capable of performing a spectrometry, with compact, lightweight equipment, cost competitive even with respect to current area monitors, as well as large and lightweight to compete with geophysical detectors.